Multiple-antenna technology is widely used in wireless communication systems, e.g. as the one illustrated in FIG. 1. For WCDMA-HSPA (Wideband Code Division Multiple Access—High Speed Packet Access) evolution, downlink 2 by 2 antenna MIMO (Multiple Input Multiple Output) was specified in 3GPP (3rd Generation Partnership Project) Release 7. Downlink (DL) MIMO was standardized for LTE (Long Term Evolution) in 3GPP Release 8. The introduction of multiple-antenna technology in the uplink is currently being discussed within 3GPP. The topics discussed include ULTD (UpLink Transmit Diversity) for WCDMA, and UL (UpLink) MIMO for LTE. The respective concepts of ULTD and UL MIMO require that UEs (User Equipment) should be provided with at least 2 TX (transmit) antennas.
Hereinafter, it is assumed that UEs are equipped with at least 2 TX antennas. For the sake of clarity, most examples herein relate to UEs provided with 2 TX antennas. The invention is, however, also valid for the cases where UEs and/or base stations (BSs) are equipped with more than 2 TX antennas and for UL MIMO beyond 2×2 antenna configuration.
UpLink Transmit Diversity
The concept of ULTD for WCDMA discussed within 3GPP includes the diversity schemes: AS (Antenna Switching) and BF (Beam Forming).
When applying antenna switching, a UE transmits from the TX antenna which for the moment has the better estimated uplink quality in terms of e.g. received uplink DPCCH (Dedicated Physical Control CHannel) power or DPCCH SINR (Signal-to-Interference-and-Noise Ratio). This in turn can be determined by the UE from the received UL TPC (Transmit Power Control) command sent by a Node B.
When applying beam forming, the UE transmits simultaneously from both TX antennas using weight factors, which are sometimes also referred to as a pre-coding vector, to maximize the estimated received power or SINR at the Node-B receiver.
The ULTD technology can, in most cases, improve the capacity and coverage of the communication system and/or reduce the UE battery consumption. There are two types of ULTD modes: Open Loop ULTD (OLTD) and Closed Loop ULTD (CLTD). In the closed loop mode, there is a specific downlink feedback channel from Node B to the UE, carrying the pre-coding vector or information to assist the generation of pre-coding vectors for ULTD. For open loop mode, there is no OLTD-specific downlink feedback channel from Node B to the UE to support the ULTD operation.
In CLTD, in order for the Node B to be able to generate the desired pre-coding information to the UE and provide it over the specific downlink feedback channel, the Node B should monitor the uplink channels. Traditionally the pre-coding vector is generated on the Node B side and is provided to the UE.
Alternatively, the Node B could send, e.g. only the CSI (Channel State Information) to the UE, which in turn could autonomously determine a suitable pre-coding vector based on the CSI.
It is realized that UL-MIMO may be a future concept also for standards such as WCDMA. Below, the transmit diversity modes OLTD, CLTD and UL-MIMO for WCDMA-HSPA will be described in further detail in a respective section.
OLTD for WCDMA-HSPA
The OLTD for WCDMA has been studied within the Release-10 time frame in 3GPP. The UE determines the transmit adaptation of the 2 TX antennas based on already existing available information, e.g. the TPC commands. The algorithms discussed for OLAS and for OLBF are described in [1].
Open Loop Antenna Switching (OLAS)
In case of OLAS, the UE typically comprises two TX antennas and a single full-power power amplifier. With the scheme described in [1], the UE TX antenna to be used is selected in the UE according to the TPC statistic:                1) Let TPC command DOWN be represented by −1 and TPC command UP by 1. Then let the UE accumulate all received TPC commands.        2) At each frame border the accumulated TPC sum is compared with 0. If the sum is larger than 0 the transmit antenna is switched.        3) If the same transmit antenna has been used for x consecutive frames the UE automatically switches antenna. x can be referred as the forced switch circle and determined according to the radio environments.        4) Every time an antenna switch occurs the accumulated TPC sum is reset to 0.Open Loop Beam Forming (OLBF)        
In case of OLBF, the UE typically comprises one power amplifier per TX antenna. With the algorithm described in [2], the UE adjusts the beam by adjusting the relative phase difference between two antennas based on the received TPCs, and could be described e.g. as follows in pseudo code:                A. The phase offset, δ can be 48 degrees, ε can be 12 degrees.        B. Let TPC command DOWN be represented by −1 and TPC command UP by +1.                    1. Initial relative phase between two transmitters Δφ=−δ/2 for the first slot (#1 slot). ε is kept zero until two TPC commands become available to the UE.            2. Apply relative phase for the next slot Δφ=Δφ+δ            3. Determine new relative phase:                            a. if TPC1>TPC2, Δφ=Δφ+ε                b. if TPC2>TPC1, Δφ=Δφ−ε                c. otherwise, no change                Note that TPC1 and TPC2 correspond to slot (1,2),(3,4), . . . , (i*2-1, i*2), where i=1 to n.                                    4. Apply relative phase for the next slot Δφ=Δφ−δ            5. Go to step 2                        
A UE which is capable of UL OLTD could in principle be configured in default mode (fixed single TX antenna) or OLTD mode (OLAS or OLBF). There is no radio link reconfiguration or transport format change of the control channels when the transmit mode of a UE is changed between the default, OLAS and OLBF mode. This means that all cells in the active set can decode the data transmitted by the UE when the transmit mode is changed within the aforementioned range. The active set is the set of cells involved e.g. in a soft/softer handover for a particular UE. For example, the cells in the active set can all participate in the reception of data from the UE and they all transmit UL TPC commands to the UE. A set corresponding to the active set may be named differently in different cellular systems.
CLTD in WCDMA-HSPA
The CLTD mode was proposed in 3GPP RAN1-61 conference [2]. In [2], an uplink closed loop transmit diversity scheme based on the explicit uplink channel estimation and CSI feedback was proposed. Within this proposal, the network controls the UE behavior and the Node B decides the weights (pre-coding vector). The simulation results in [2] show that the average throughput gain reaches 14% in Pedestrian A channel (3 km/h) and up to 10% in Vehicular A (30 km/h) channel. Thus, CLTD can be a valuable complement, which should be further considered for improving the HSPA uplink. The CLTD mode includes CLBF (Closed Loop Beam Forming) and CLAS (Closed Loop Antenna Switching). CLBF implies that a desired signal is multiplied with a pre-coded vector determined by the network. CLAS implies that the signal is only transmitted on one antenna at the time, which antenna is selected by the network. Theoretically, CLBF could be used to transmit two streams. However, typically when discussing CLBF for e.g. WCDMA, only one stream is implied.
A UL CLTD UE can be configured in default mode, OLTD (OLAS or OLBF) mode and CLTD (CLAS or CLBF) mode. As previously mentioned, a cell in the active set can decode the data transmitted by a UE when the transmit mode of a UE is changed between the default, OLAS and OLBF mode. When the primary DPCCH is always pre-coded with the same precoding vector as the data part and the primary DPCCH configuration (e.g. spreading code, pilot sequences, transport format) is not changed due to the transmit mode change of a UE, a cell in the active set can always decode the data based on the estimated channel from the primary DPCCH when the transmit mode of a UE is changed within the range of default, OLAS, OLBF, CLAS and CLBF mode without any reconfiguration of the uplink receiver of this active cell. However, some changes may be needed in receiver configuration for some active cell, such as e.g. that the serving cell should be configured to generate the precoding vector for a UE in CLAS/CLBF mode.
UL MIMO in WCDMA-HSPA
The Uplink Multiple Input Multiple Output technology (UL MIMO) is another advanced technology to improve the uplink data rate. With UL MIMO, the UE can transmit either a single data stream or dual data streams in the uplink.
For LTE, UL MIMO for up to 4 TX antennas in the UE is being specified [3]. For WCDMA-HSPA, UL beam-forming has not been standardized and there is no hardware update for a ULBF UE to support UL MIMO. In a typical WCDMA-HSPA Node B, there are two receive antennas. In the future, network deployments with more receive antennas and more advanced receiver structures may become increasingly common. This means that there can be even larger gain from UL MIMO in future network deployments.
A UE which is capable of UL MIMO can be configured in: default, OLTD (OLAS or OLBF), CLTD (CLAS or CLBF), and UL MIMO (single-stream or dual-stream MIMO) mode. The characteristics of data reception and decoding for signals received from an “UL MIMO UE” is similar to that of a “CLTD UE”, when the transmit mode of the UL MIMO UE is changed within the modes: default, OLAS, OLBF, CLAS and CLBF. When the primary DPCCH is pre-coded with the same pre-coding vector as the primary data stream and the secondary DPCCH is pre-coded with the same pre-coding vector as the secondary data stream in UL dual-stream MIMO mode, a cell in the active set can decode at least the primary data stream based on the estimated channel of the primary DPCCH, given that the primary DPCCH configuration is not changed at any transmit mode change. However, in order to generate the pre-coding vector for a UE in any of the modes CLAS, CLBF, UL single-stream MIMO or dual-stream MIMO, the serving cell should be configured accordingly. When a UE is configured to dual stream MIMO mode from any other transmit mode, the receiver of a cell in the active set should be configured for dual-stream MIMO mode in order to be able to decode both data streams. The mode “single stream MIMO” can be equivalent to the mode “CLBF”.
UE Transmit Mode Configuration
It can be foreseen that the UE transmit mode configuration in a WCDMA system could and would be controlled by the RNC (Radio Network Controller). Reasons for this is that the RNC typically has access to information on the capabilities of both the UE and the serving Node B, the non-serving Node Bs, and other neighboring nodes. One example of such information which is valuable is whether the non-serving Node B is UL CLTD/MIMO-capable or not. Further, the RNC has access to measurement reports from both the serving Node B and non-serving Node Bs. Further, the RNC can notify both the serving Node B and the non-serving Node Bs of the currently configured transmit diversity mode of a specific UE.
Having access to the information described above, the RNC can configure the UE in a proper TD (Transmit Diversity) mode, e.g. for achieving a smooth/seamless handover, when a UE is to be handed over between an UL CLTD/MIMO-capable Node B and a legacy Node B, which is not UL CLTD/MIMO-capable. Further, the RNC can also configure the UE in a proper TD mode, based on e.g. i) the UE capability, ii) the capabilities of the serving and non-serving Node Bs and iii) the uplink measurements, in order to improve the overall system performance. A signaling scheme illustrating an RNC controlled transmit mode configuration of a UE is shown in FIG. 2.
However, it is realized that a UE TD mode control based only on the previously listed information and considerations will not be able to adapt adequately to certain changes, e.g. fast changes in uplink traffic load, since the RNC does not have access to such information.